1. Field of the Invention
The present invention relates generally to sealing elements for hydraulic and pneumatic machine elements. Specifically, the present invention relates to a buffer seal for providing a fluid seal between relatively moving parts, such as a piston or rod moving within a bore and, in particular, the present invention relates to a buffer seal providing a controlled pressure at a primary seal.
2. State of the Art
Seals adapted to provide a fluid seal between two relatively moving machine elements are well known in the art. For example, one or more sealing elements are commonly used to provide a fluid seal between a piston or rod moving within, and relative to, a bore extending through a housing or other machine element. Although a single seal may be disposed between an outer cylindrical surface of the piston or rod and an inner cylindrical surface of the bore, it is a common practice to employ a combination of two or more sealing elements (i.e., a seal assembly) to provide a robust fluid seal between the two relatively moving machine elements.
A conventional seal assembly is shown in FIG. 1. The conventional seal assembly 5 is configured to provide a fluid seal between, for example, a rod 11 of a first machine element 10 moving within a bore 21 of a second machine element 20 and relative thereto. Although the first machine element 10 is shown including a rod 11 and the second machine element 20 is shown including a bore 21, it will be appreciated by those of ordinary skill in the art that each of the first and second machine elements 10, 20, respectively, may be comprised of multiple machine parts or elements. For example, the rod 11 of first machine element 10 may comprise two or more separately formed parts that are subsequently attached to one another to form the assembled rod 11.
The rod 11 and bore 21 of the first and second machine elements 10, 20, respectively, are generally mutually concentric to a central longitudinal axis 15. Further, the rod 11 of first machine element 10 and the bore 21 of second machine element 20 are cooperatively dimensioned to enable the rod 11 and bore 21 to slide and/or rotate relative to one another. A clearance gap 90 between an outer cylindrical surface 12 of the rod 11 and an inner cylindrical surface 22 of the bore 21 enables relative motion between the rod 11 and bore 21. It should be noted that in FIG. 1 the size of the clearance gap 90 has been exaggerated for clarity; however, those of ordinary skill in the art will understand that such a clearance gap 90 may, in practice, be extremely small in comparison to the dimensions of the rod 11 and bore 21. For example, the clearance between the outer cylindrical surface 12 of the rod 11 and the inner cylindrical surface 22 of bore 21 may be on the order of a few thousandths of an inch or less.
Relative motion between the first machine element 10 and the second machine element 20 may be the result of the rod 11 traveling longitudinally along axis 15 through the bore 21 of a relatively stationary second machine element 20 or the result of the second machine element 20 traveling longitudinally along axis 15 over a rod 11 of a relatively stationary first machine element 10. Alternatively, relative motion between the first and second machine elements 10, 20 may be the result of longitudinal travel of both the first and second machine elements 10, 20, respectively, along axis 15. Also, relative motion between the first machine element 10 and second machine element 20 may be the result of relative rotary motion between the first and second machine elements 10, 20, or the result of a combination of relative longitudinal motion and relative rotary motion therebetween.
The seal assembly 5 comprises a wiper 30, a primary seal 50, and a buffer seal 70. The wiper 30 is disposed in an annular groove or gland 40, formed about the circumference of the inner cylindrical surface 22 of the bore 21 extending through second machine element 20. Similarly, the primary seal 50 is disposed in a gland 60 formed about the circumference of the inner cylindrical surface 22 of bore 21 and the buffer seal 70 is disposed in a gland 80 formed about the circumference of the inner cylindrical surface 22 of bore 21. Although the glands 40, 60, 80 are shown disposed about the inner cylindrical surface 22 of bore 21, and the wiper 30, primary seal 50, and buffer seal 70 disposed therein, respectively, it should be understood by those of ordinary skill in the art that one or more of the glands 40, 60, 80 could be disposed about the circumference of the outer cylindrical surface 12 of the rod 11 of first machine element 10.
The wiper 30 is a generally ring-shaped member disposed in the gland 40. The wiper 30 is adapted to prevent the ingress of solid particulates and other contaminants into the clearance gap 90 from the ambient side 92 of the seal assembly 5 (i.e., the end of the seal assembly 5 exposed to ambient environmental conditions) to a region 94 between the wiper 30 and the primary seal 50 where such contaminants could potentially damage or inhibit proper functioning of the primary seal 50. Wipers for use in hydraulic and pneumatic fluid sealing applications are well known in the art.
The purpose of the primary seal 50 disposed in gland 60 is to prevent the flow of fluid from the system side 98 of the seal assembly 5 (i.e., the end of the seal assembly 5 exposed to pressurized fluid) and through the clearance gap 90 to the ambient side 92 of the seal assembly 5. Any leakage of fluid past the primary seal 50 to the ambient side 92 of the seal assembly 5 may compromise system fluid pressure and operation. For example, the proper functioning of a hydraulically-actuated piston used to apply a load over a specified distance (e.g., hydraulic cylinders on construction equipment) depends upon the maintenance of system fluid pressure. Seals adapted for use as a primary seal 50 in hydraulic and pneumatic fluid sealing applications are well known in the art. These seals may be statically loaded or, alternatively, dynamically loaded during operation.
A statically loaded seal typically comprises a generally ring-shaped, resiliently elastic body exhibiting a geometry adapted to provide a necessary sealing force. For example, with reference to FIG. 1, a statically loaded primary seal 50 may comprise a resilient body disposed in the gland 60 and having a geometry such that, upon insertion of the rod 11 into the bore 21 of the first and second machine elements 10, 20, respectively, the resilient body deforms or compresses and exerts radially inward-directed forces about the circumference of the outer cylindrical surface 12 of the rod 11. The resilient body further exerts corresponding radially outward-directed forces about the circumference of the outer wall 61 of the gland 60 and/or exerts longitudinally directed forces about the periphery of one or both of the side walls 62, 63 of the gland 60. The forces exerted by the resilient body against the outer cylindrical surface 12 of the rod 11 and one or more of the walls 61, 62, 63 of the gland 60 prevent, or at least substantially inhibit, the flow of fluid around the resilient body. Therefore, fluid pressure on such a statically loaded seal is unnecessary for the statically loaded seal to maintain a fluid seal and, further, excessive system pressure on a statically loaded seal can cause high friction, heat generation, increased wear, and reduced seal life.
A dynamically loaded seal typically comprises a generally ring-shaped, resiliently elastic body. However, the resilient body is configured to provide a necessary sealing force, or at least a significant portion of the sealing force, when subjected to system fluid pressure. The resilient body may include a structure, such as a cylindrical lip, adapted to impinge against a surface of a machine element when acted upon by pressurized fluid. By way of example with reference to FIG. 1, a dynamically loaded primary seal 50 may comprise a resilient body disposed in the gland 60 and configured to impinge against one or more of the walls 61, 62, 63 of the gland 60 about the periphery thereof, respectively, to provide a fluid seal between the resilient body and the gland 60. The resilient body further includes a feature, such as a lip structure extending about a circumference thereof as noted above, that deforms and exerts radially inward-directed forces about the circumference of the outer cylindrical surface 12 of the rod 11 when acted upon by pressurized fluid. Thus, proper functioning of such a dynamically loaded primary seal 50 requires that a minimum threshold system fluid pressure be maintained. Although the threshold fluid pressure must be maintained for proper functioning of a dynamically loaded seal, excessive pressure on such a seal can lead to high friction, heat generation, increased wear, and reduced seal life as indicated above for a statically loaded seal.
Although statically loaded and dynamically loaded seals were described separately above, those of ordinary skill in the art will understand that, in practice, fluid seals often exhibit a combination of loading characteristics. For example, a statically loaded seal will typically experience at least some dynamic loading during operation and a dynamically loaded seal will typically exhibit at least some static loading. Thus, a primary seal 50 may include geometry adapted to exert sealing forces about the circumference of the outer cylindrical surface 12 of the rod 11 and about the periphery of one or more of the walls 61, 62, 63 of the gland 60 and may further include structure, such as a lip as described above, that provides additional sealing forces when subjected to system fluid pressure.
The primary seal 50 may also include an anti-extrusion ring 52 configured to prevent the body of the primary seal 50 which, as suggested above, is typically a compliant material, from being extruded into the clearance gap 90 as a result of high system pressure or relative movement of the primary seal 50 within gland 60, or a combination thereof. The anti-extrusion ring 52 is typically constructed of a material relatively harder and more rigid than the material used to construct the body of the primary seal 50.
As noted above, the conventional seal assembly 5 also includes a buffer seal 70. The buffer seal 70 is a generally ring-shaped body disposed in a third gland 80 formed about the circumference of the inner cylindrical surface 22 of the bore 21 extending through second machine element 20. Although shown in FIG. 1 as having a generally rectangular cross-section, such a conventional buffer seal 70 may have any suitable cross-sectional shape or configuration as known in the art. Also, the buffer seal 70 may include an anti-extrusion ring 72 as described above.
The buffer seal 70 is disposed between the system side 98 of the seal assembly 5 and the primary seal 50. The primary function of the buffer seal 70 is to prevent extreme system pressure conditions from acting upon the primary seal 50 and causing failure of, or damage to, the primary seal 50. Such an extreme pressure condition may include, for example, a high-pressure spike propagating through the system side 98 of the seal assembly 5 that impacts the seal assembly 5. Also, if excessive pressure builds up in the region 96 between the buffer seal 70 and the primary seal 50 (i.e., xe2x80x9cback pressurexe2x80x9d) the buffer seal 70 should vent the back pressure to the system side 98 of the buffer seal 70. Further, the buffer seal 70 should prevent solid particulates and other contaminants on the system side 98 of the seal assembly 5 from reaching the primary seal 50 and causing damage to, or failure of, the primary seal 50.
Although seal assemblies comprised of a wiper, primary seal, and buffer seal, such as the seal assembly 5 shown in FIG. 1, are well known in the art, such seal assemblies are prone to failure due to deficiencies in operation of the buffer seal. A common problem with conventional buffer seals is that the buffer seal simply does not sufficiently dampen high-pressure spikes propagating through the system. Conventional buffer seals 70 exhibit a deformed or compressed axial thickness that is less than an axial thickness of the gland 80 in which the buffer seal 70 is retained, such that a gap 74 exists between the side walls 82, 83 of the gland 80 and the buffer seal 70. Thus, the buffer seal 70 is allowed to float within the gland 80, enabling the buffer seal 70 to travel axially along axis 15 within the gland 80 between the side walls 82, 83 thereof and further enabling the buffer seal 70 to rotate or tilt within the gland 80. It is believed that the ability of the conventional buffer seal to axially travel and/or tilt within its mating gland is, at least in part, responsible for the failure to dampen high-pressure spikes.
The freedom to axially travel within a gland 80 can be especially problematic for conventional buffer seals 70 that, in addition to exhibiting sealing contact with the rod 11 of first machine element 10, exhibit sealing contact at only the contact interface between the buffer seal 70 and the side wall 83 of the gland 80 nearest the primary seal 50. A conventional buffer seal may be specifically designed to provide such a sealing contact with only the side wall 83 of the gland 80 nearest the primary seal 50, in which case the buffer seal 70 would not break sealing contact with the side wall 83 unless or until fluid accumulates in the region 96 between the buffer seal 70 and primary seal 50. However, travel of the rod 11 of first machine element 10 relative to the bore 21 of second machine element 20 may itself cause the conventional buffer seal 70 to fail. Travel of the rod 11 in a direction away from the primary seal 50 and toward the buffer seal 70 can axially displace the buffer seal 70 relative to its associated gland 80, thereby xe2x80x9cpullingxe2x80x9d the buffer seal 70 away from the side wall 83 of the gland 80 and breaking sealing contact therewith, enabling fluid to flow around the buffer seal 70 and potentially allowing high-pressure spikes to be transmitted to the primary seal 50.
Failing to adequately exclude system contaminants from the primary seal 50 is another problem exhibited by conventional buffer seals 70. The lack of a suitable volume or region in which solid particulates can collect or be trapped, as well as the inability of the anti-extrusion ring 72, if present, to exclude solid particulate matter, are believed to contribute to the inability of conventional buffer seals 70 to adequately exclude system contaminants from the primary seal 50.
A further problem with conventional buffer seals 70 is the inability to regulate the fluid pressure in the region 96 between the buffer seal 70 and the primary seal 50 (i.e., the back pressure). The inability to regulate or relieve the back pressure results in pressure trapping. Pressure trapping occurs when a high fluid pressure within the region between the buffer seal 70 and the primary seal 50 builds up during operation but the buffer seal 70 is unable to relieve this high back pressure or to maintain the back pressure at or below a desired operating pressure. Excessively high back pressure can cause a number of deleterious effects, including increased friction between the primary seal 50 and a relatively moving body, increased heat generation, increased wear, and reduced seal life. If the primary seal 50 is a dynamically loaded seal, the inability of the conventional buffer seal 70 to regulate the back pressure may also result in a condition in which the fluid pressure acting on the dynamically loaded primary seal 50 is insufficient for the primary seal 50 to maintain a fluid seal. Also, fluid trapped in the region 96 between the buffer and primary seals 70, 50 may itself provide a medium for propagating high-pressure spikes to the primary seal 50.
Thus, a need exists in the art for a buffer seal adapted for use in a seal assembly having a primary seal, the buffer seal being able to effectively and repeatably dampen out high-pressure spikes such that these high-pressure spikes do not impact the primary seal. A need also exists for a buffer seal capable of trapping solid particulates and other contaminates to exclude such contaminates from the primary seal. Further, a need exists for a buffer seal configured to regulate the back pressure, such that a minimum threshold back pressure can be maintained to dynamically load a primary seal while preventing the build up of excessively high back pressure that could be imparted to the primary seal.
The present invention encompasses a number of embodiments of a buffer seal adapted for use in a seal assembly including at least a primary seal. The seal assembly, including a buffer seal according to the invention, may be used to provide a fluid seal between two relatively moving machine elements, such as a piston or rod of a first machine element moving within a cylindrical bore of a second machine element.
An exemplary embodiment of a buffer seal according to the invention may comprise a sealing element and a biasing element. The sealing element is a generally ring-shaped body having at least one surface configured to contact a surface of the first machine element and to provide a fluid seal thereagainst and further having at least one surface configured to contact a surface of the second machine element and to provide a fluid seal thereagainst. The biasing element is a generally ring-shaped body constructed of a resiliently deformable material configured to impart a biasing force or sealing force against the sealing element. The material and volume of the biasing element are selected to provide a biasing force sufficient to simultaneously maintain the respective surfaces of the sealing element in contact with the surfaces of the first and second machine elements and, further, to minimize axial travel and tilting of the buffer seal within its associated gland, irrespective of the direction of relative travel between the first and second machine elements, thereby providing a robust fluid seal and dampening high-pressure spikes prior to impact with the primary seal.
In further embodiments of the invention, the sealing element may be configured to inhibit the migration of contaminants and debris from the system side of the seal assembly and around the buffer seal to the primary seal. In one embodiment, the sealing element may include a structure configured to act as a wiper or scraper against a surface of one of the first and second machine elements. In another embodiment, the sealing element may include a relief volume in which debris can be collected or trapped.
In yet another embodiment of a buffer seal according to the invention, the buffer seal may be configured as a pressure relief valve to provide a controlled back pressure in the region between the buffer and primary seals. The material, shape, and/or volume of the biasing element are selected such that, if the back pressure exceeds a specified threshold pressure, the buffer seal will release a controlled volume of fluid in order to restore the back pressure to the threshold pressure. To facilitate the release of fluid, the buffer seal may include a biasing element configured with a fluid path to allow fluid to flow around the buffer seal. Such a buffer seal configured to provide a controlled back pressure may be useful for seal assemblies including a predominantly dynamically loaded primary seal requiring a minimum operating pressure for proper functioning, as well as for seal assemblies including a predominantly statically loaded primary seal.